This invention relates to the field of combustion, and, in particular, to gas turbines that are driven by the exhaust from combustors that burn hydrocarbon fuels.
In combustors used to produce gases that drive turbines, there are two problems that have not yet been completely solved. First, at the customary high velocities of the gas streams in such combustors, the combustion flame can blow out. Secondly, the NOx in the exhaust stream can be as high as 25 ppm with gas fuel, and higher with distillate fuel.
It has been known, in the prior art, that as little as 2% of hydrogen, by volume, when mixed with the fuel, can stabilize the flame and prevent blowout.
It has also been known, in the prior art, that if the amount of hydrogen in the gas stream is increased to about 15%, by volume, the level of NOx can be reduced to about 2 ppm.
The present invention therefore provides a means of supplying hydrogen to the fuel used to power a combustor that drives a gas turbine. By supplying hydrogen in the necessary amount, the invention provides a combustion system in which the likelihood of blowout is minimized, and in which the level of NOx is low.
The present invention includes a combustion system which comprises a steam reformer, a combustor, and a source of fuel, which may be gas or liquid. A portion of the incoming fuel is diverted to the steam reformer through a booster pump, while the remainder of the fuel passes directly into the combustor. A source of steam is connected to the steam reformer. The steam reformer produces an effluent which is connected to the inlet of the combustor, so as to mix with the unreformed fuel. Because a significant portion of the effluent from the steam reformer is hydrogen, the incoming fuel-air mixture at the inlet end of the combustor will include hydrogen.
In the preferred embodiment, one chooses the amount of fuel diverted to the steam reformer according to a stoichiometric calculation, such that the volume of hydrogen in the fuel, at the inlet of the combustor, is up to about 15%. The result is that the combustor produces a relatively small amount of NOx.
The system may be automatically controlled by controlling a pump which supplies fuel to the steam reformer. When the concentration of NOx in the outlet stream of the combustor exceeds a predetermined level, the speed of the pump is increased, so as to increase the fuel flow to the reformer, thereby increasing the amount of hydrogen appearing at the inlet of the combustor. This control is preferably accomplished with a microprocessor that is connected to control the pump.
The invention also includes the method of operating a combustor for a gas turbine, wherein a portion of the incoming fuel is diverted to a steam reformer, the remainder passing directly to the combustor, and wherein the effluent from the steam reformer is combined with unreformed fuel entering the combustor.
The present invention therefore has the primary object of providing a combustion system for a gas turbine, wherein the combustion system produces very small amounts of NOx.
The invention has the further object of providing a combustion system for a gas turbine, wherein the combustion flame is not likely to blow out.
The invention has the further object of providing an efficient method of operating a combustor for use in a gas turbine system.
The invention has the further object of combining a steam reformer with a combustor to produce a combustion gas for driving a turbine, wherein the combustion gas has a low level of NOx.
The invention has the further object of providing a combustion system which automatically maintains a desired level of NOx in its outlet stream.
The reader skilled in the art will recognize other objects and advantages of the invention, from a reading of the following brief description of the drawing, the detailed description of the invention, and the appended claims.